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Intelligent reflecting surface (IRS) is a promising and disruptive technique
to extend the network coverage and improve spectral efficiency. This paper
investigates an IRS-assisted Terahertz (THz) multiple-input multiple-output
(MIMO)-nonorthogonal multiple access (NOMA) system based on hybrid precoding in
the presence of eavesdropper. Two types of sparse RF chain antenna structures
are adopted, i.e., sub-connected structure and fully connected structure.
Cluster heads are firstly selected for transmissions, and discrete phase-based
analog precoding is designed for the transmit beamforming. Subsequently, based
on the channel conditions, the users are grouped into multiple clusters, and
each cluster is transmitted by using the NOMA technique. In addition, a low
complexity zero-forcing method is employed to design digital precoding so as to
eliminate interference between clusters. On this basis, we propose a secure
transmission scheme to maximize the sum secrecy rate by jointly optimizing the
power allocation and phase shifts of IRS under the constraints of system
transmission power, achievable rate requirement of each user, and IRS phase
shifts. Due to multiple coupled variables, the formulated problem leads to a
non-convex issue. We apply the Taylor series expansion and semidefinite
programming to convert the original non-convex problem into a convex one. Then,
an alternating optimization algorithm is developed to obtain a feasible
solution of the original problem. Simulation results are demonstrated to
validate the convergence of the proposed algorithm, and confirm that the
deployment of IRS can significantly improve the secrecy performance.
